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PSSM1 causes muscle cramping and tying up

Full name

Polysaccharide storage myopathy 1

Breeds known to be affected

Various, including Quarter Horse and Quarter Horse derivatives, and many draught breeds. PSSM1 has not been identified in purebred Arabians, Thoroughbreds or Standardbreds.

Inheritance pattern

Autosomal dominant


PSSM1 causes a build-up of abnormal sugars in muscle. This is one of the causes of tying up, with clinical signs that include muscle twitches, stiffness, sweating, reluctance to move and painful cramps. Symptoms can vary widely in severity and age of onset. PSSM1 appears to be quite an old mutation so it is found in many breeds including Quarter Horses and draft breeds. It is a complex disorder that can often be controlled with changes to diet and exercise management.

PSSM1 is associated with a mutation in the GYS1 gene and is inherited in a dominant fashion, so a horse only needs to carry one copy (PSSM1/n) to show symptoms. There is some evidence that homozygous horses (PSSM1/PSSM1) are more severely affected than heterozygotes. Please note that this test only detects this one specific type of tying up, and horses may still exhibit signs of tying up even if they are not positive for PSSM1.

Interpretation of results

n/n: Horse does not carry the mutation associated with PSSM1.

PSSM1/n: Horse has one copy of the mutation associated with PSSM1.

PSSM1/PSSM1: Horse has two copies of the mutation associated with PSSM1.


McCue et al. Glycogen synthase (GYS1) mutation causes a novel skeletal muscle glycogenosis. Genomics 2008; 5:458-466 doi:10.1016/j.ygeno.2008.01.011

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